Marinate tuna in soy sauce or soy sake marinade for 10 minutes. Remove from marinade and pat dry with paper towels before using. (click for guide)
Cover the sheet of nori in a thin even layer of rice, filling all the way out to the edges. (click for guide)
Flip the sheet of nori over so the rice is face down on a bamboo rolling mat that is wrapped in plastic. (click for guide)
Arrange the fillings (soy marinated tuna, avocado, and green onion) in even lines, 1 inch from the edge along the longer side of the nori sheet. (click for guide)
Place the plastic-wrapped rolling mat over the top of the roll and press the roll into a consistent shape. (click for guide)
Before cutting the roll, coat it in black tobiko. Spoon some tobiko onto the top of the roll and use the back of the spoon to smooth it down the sides of the roll as well. Try to get as even of a coating as possible, it may be a bit messy. (I typically don't coat the bottom of the roll but that's an option depending on how you plan to plate it)
Place the roll on a cutting board and grab your sharpest knife. Run the knife under water for a second, then shake off any excess. Cut the roll into 8 equal pieces using a gentile sawing motion with the knife, running the knife under the water between every 2-3 cuts to prevent sticking. (click for guide)